1. Field of the Invention
The present invention relates to the field of High Intensity Discharge (HID) lamps, and more particularly, to electronic ballast of such lamps.
2. Description of the Related Art
FIG. 1A is a schematic diagram of a conventional electronic ballast of an HID lamp. In this ballast, a microprocessor (MPU) reads a scaled down voltage of the HID lamp, and outputs a pulse width modulation (PWM) signal to a power converter switch driver, which drives a power converter switch. The MPU varies the duty cycle of the PWM signal in accordance with the scaled down voltage of the HID lamp, and may set the duty cycle based on values in a lookup table of the MPU, for example. The power converter switch provides power to the HID lamp in accordance with the PWM signal.
The power outputted from the ballast to the HID lamp is a function of the duty cycle of the PWM signal. However, due to component tolerances of the ballast, such as a voltage divider resistor tolerance, a tolerance of an analog to digital converter, a power inductor tolerance, and circuit delay, the output power can widely vary from one ballast to another. For example, the output power of a 70 W ballast can vary between 60 W and 80 W. Thus, the output of the ballast is not only a function of the duty cycle of the PWM signal, but also is a function of the component tolerances.
To minimize the output power variations among ballasts, components with tight tolerances can be used. However, a disadvantage of such a design is the associated increase in cost.
FIG. 1B is a schematic diagram of a second type of conventional electronic ballast of an HID lamp. In this ballast, a MPU does not output a PWM signal directly to a power converter switch driver, as in the ballast shown in FIG. 1A. Instead, the MPU outputs a PWM signal to an input of an operational amplifier. The duty cycle of the PWM varies in accordance with the scaled voltage of the HID lamp, and may be set, for example, based on values in a lookup table of the MPU. The second type of the conventional electronic ballast has the same component tolerance issue as the first type of conventional electronic ballast. However, to minimize the output power variation, a potentiometer is connected to a second input of the operational amplifier, and is used to trim the output of the power converter switch.
FIG. 1C is a schematic diagram of the second type of conventional electronic ballast when it is in a trimming mode. To trim the output of the second type of ballast, the ballast output is connected to a fixed resistor, rather than an HID lamp. Typically, the resistance of the resistor corresponds to an HID lamp impedance at a nominal wattage. An operator measures the output power of the ballast, and turns the potentiometer to trim the output power until he or she determines that it has reached an acceptable value.
A disadvantage of this ballast is that the potentiometer can be adjusted to compensate for error at only one set point, typically the impedance at nominal lamp wattage. However, the lamp impedance is not a constant value during the entire time the lamp is in operation. Thus, the MPU cannot provide an accurate ballast output throughout the entire time the lamp is in operation.